Stratolaunch Systems, a unit of Allen's privately owned Vulcan Aerospace, allowed reporters to have a first look at the nearly finished aircraft few days ago.
The six-engine plane will have the wingspan of 385 feet (117 m). It will be larger than Howard Hughes' 1947 H-4 Hercules, known as the "Spruce Goose," and the Antonov An-225, a Soviet-era cargo plane originally built to transport the Buran space shuttle.
Allen's vision is completely different from that of Elon Musk's SpaceX, Jeff Bezos' Blue Origin, Richard Branson's Virgin Galactic and other companies.
Musk is planning to take people to Mars. Bezos is focused on developing low-cost, reusable rockets with the goal of moving energy-intensive, while Branson wants to earn money by space tourism and a small satellite launcher.
Allen's approach is advantageous as it will allow him to position the plane so satellites can be directly delivered into very precise orbits and do so quickly, without launch range scheduling issues and weather-related delays.
The Stratolaunch plane looks like a twin-fuselage craft that incorporates engines, landing gear, avionics and other parts from a pair of Boeing 747 jets coupled with a frame, wings and skin handmade of lightweight composites. The plane is designed and built by Northrop Grumman Corp's Scaled Composites.
Stratolaunch is also planning a similar service for satellites, particularly the low-Earth orbiting multi-hundred member constellations, which are currently under development by companies including SpaceX and Google's Terra Bella to provide internet access, Earth imagery and other data.
These satellite networks are based on low-cost spacecraft and are the fastest-growing segment of the global satellite industry.